This invention relates to a medical diagnostic ultrasound system and method for harmonic imaging. In particular, the invention relates to generation of waveforms for use with electrostatic transducers.
Electrostatic transducers, such as micro-machined capacitance based electrostatic transducers, offer many advantages over traditional ceramic transducers. For example, electrostatic transducers may be cheaper to manufacture, allow higher manufacturing yields, provide more size and shape options, use non-toxic materials and have higher bandwidth. However, electrostatic transducers generate harmonic signals when excited with a waveform at a fundamental frequency. The output force is proportional to the square of the applied voltage signal. Additional non-linearities may also exist, such as non-linearities within the membrane and non-linear motion (e.g., bottoming out of the membrane).
For imaging based on the non-linear response at a region of interest in a patient, harmonic information is isolated or combined with fundamental information. Preferably, little or no harmonic information is generated by the system, providing harmonic information based more on the response of the tissue or any contrast agents. To counter undesired generation of harmonic information through propagation, the excitation waveform has a shaped envelope, such as the Gaussian pulses disclosed in U.S. Pat. No.5,740,128. Additionally, the excitation waveform may be pre-distorted. See U.S. application Ser. No. 09/061,014, filed Apr. 15, 1998. The ""014 application also suggests that xe2x80x9cin order to correct for non-linearity introduced in the transmit channel, the electrical waveforms are pre-distortedxe2x80x9d (pg. 13, lines 12-13). The application also notes that the transducer is included within the transmission channel (pg. 12, lines 26-29). However, non-linear signal generation problems associated with specific types of transducers are not discussed.
The present invention is defined by the following claims, and nothing in this section should be taken as a limitation on those claims. By way of introduction, the preferred embodiment described below includes a method and system for generating transmit pulses for use with an electrostatic transducer. The excitation waveforms are pre-distorted to account for the non-linear output of the electrostatic transducer.
In one aspect, two excitation waveforms are applied to a multiple element electrostatic transducer array. One of the excitation waveforms compensates for the non-linear output due to the application of the other excitation waveform.
In another aspect, the excitation waveforms discussed above are applied to a transducer array characterized by a force/electrical transduction process with a non-linear response.
In yet another aspect, a force on a multiple element electrostatic transducer array is measured. An excitation waveform is applied to the transducer array. A bias voltage is also applied where the bias voltage is responsive to the measured force.
Further aspects and advantages of the invention are discussed below in conjunction with the preferred embodiments.